The visual system of the Syrian hamster provides a useful model for study of behavioral changes after brain lesions, and the processes underlying alterations, sparing or recovery of function after brain damage. Particularly striking changes in the adult animal are the loss of visually guided orienting movements after bilateral ablation of the superior colliculus, with a sparing of pattern vision, and the contrasting effects of visual-cortex ablation (loss of pattern vision, with spared orienting). Yet if either lesion is inflicted in the neonatal animal, the behavioral defects are much less severe or absent. Abnormal neuroanatomical connections develop after the early lesions, some of which appear to play an important role in sparing or alteration of function. We are investigating the factors which underly this neuroanatomical plasticity, the critical periods during which they operate in the course of postnatal development, and the relationship of these factors to normal ontogenesis. We employ experimental techniques for tracing of axonal connections in the brains of animals which have suffered neonatal lesions. Comparisons are made with normal controls. In addition, electrophysiological techniques are employed primarily to study the topographic organization of central representations of the retina. Videotape analysis is used to aid the study of visually guided behavior of hamsters with early brain lesions, and to study behavioral effects of additional brain surgery, after these animals have reached adulthood.